Silvopastoral systems are ancient farming systems in the world, consisting of the combination of
a woody component (trees or shrubs) and crops and/or animals within the same land-management unit. In various European Union (EU) countries, the possibility of using sewage sludge as a fertilizer is under consideration as a viable method of disposal, considering the increase in sewage-sludge production in recent years and the restrictions imposed by European policy on the usual methods of disposal. The concern is the concentration of heavy metals, which can reach humans through the food chain. In Spain, R.D. 1310/1990, as well as European Directive 86/278, limit the total in-soil heavy-metal concentration, but not the solubility changes, which directly affect plant absorption and leaching of heavy metals throughout the soil profile. The objective of this experiment was to compare, in a silvopastoral system over a period of 3 years, the effect of applying three doses of sewage sludge combined with and without liming, on total and available soil Zn and Cu and their concentration in plants. Liming did not affectct Zn and Cu availability; however, sewage sludge increased Zn and Cu availability, though total in-soil Zn was increased only in November 2000. In-plant Zn concentration was increased by sewage sludge in the last 2 years of the study. In all cases, the quality of forage obtained and measured with regard to the concentrations of Zn and Cu was adequate for animal consumption. With respect to sewage-sludge application as a fertilizer, the management of heavy-metal availability must be included in the policy, because environmental risk could then be adequately evaluated
